This invention relates to an improved gas phase process for preparing pyridine and/or alkyl-substituted pyridines in high yields of main product, e.g., pyridine, and, preferably, with low or desirably regulated amounts of side products, e.g., often the picolines, by reacting at least one carbonyl compound and ammonia in the presence of a silica/alumina catalyst. One particularly preferred process is the preparation of pyridine by the gas phase catalytic condensation of acetaldehyde, formaldehyde and ammonia.
Such reactions have hitherto been carried out over a variety of catalysts containing silica and alumina, both crystalline and amorphous, and using a variety of reactant combinations, e.g., in addition to aldehydes, ketones and ammonia, also alcohols, water, etc.
Predominantly, the prior art catalysts have been amorphous in structure. See, e.g., U.S. Pat. Nos. 3,272,825; 3,946,020; 2,807,618; and 4,089,863; Japanese Patents Nos. 76-63,176; 71-41,546; 69-32,790; 80-151,558; and 80-151,559; Great Britain Patent No. 1,141,526; German Patent No. 2,203,384; and East German Patent No. 130,784, among many others. Such amorphous catalysts have been employed in both fixed and fluidized bed reactors without any significant differences in product yields and/or ratios of main products to side products. The yields of desired products have heretofore been unsatisfactory.
On the other hand, crystalline aluminosilicate catalysts, including those having a constraint index of about 1 to about 12 and a silica to alumina ratio of greater than about 10-12, have also been utilized in such reactions. See, e.g., U.S. Pat. No. 4,220,783 and U.S. Pat. No. 3,728,408 (e.g., column 25, line 23). However, these reactions have been carried out only in fixed catalyst beds. Once again, the resultant yields of desired products have been unsatisfactory. Moreover, in order to optimize the ratio of desired product to undesired side products, there is invariably a concomitant decrease in overall product yield; conversely, if the overall product yield is increased, there is a concomitant decrease in ratio of desired product to undesired product. For example, for the prior art reactions of acetaldehyde, formaldehyde and ammonia, the total molar yield of pyridine and .alpha.- and .beta.-picolines, based on 2 moles of acetaldehyde reactant, varies within the range of 35-72 molar percent while the weight ratio of desired pyridine to undesired picoline side products, expressed as the weight ratio of pyridine to .beta.-picoline, varies inversely, in the range of 1.2-3. Yields have been somewhat higher in the crotonaldehyde/formaldehyde/ammonia system but unsatisfactory ratios of desired to undesired products and unsatisfactory overall results are still obtained. See, e.g., German patents Nos. 1,931,945 and 2,051,316. There remains a need to improve these results in favor of the desired product which is pyridine.
Heretofore, the nature of the catalyst bed (fixed or fluidized or otherwise movable), has not been a factor in increasing yield of desired product. In general, fixed beds are employed in these catalytic reactions unless the additional expenditures associated with a fluidized bed are warranted by system-specific considerations. As is well known, (See, e.g., "Fluidized Bed Technology" by Zenz, in Kirk-Othmar, Encyclopedia of Chemical Technology, 2nd Edition, Volume 9, page 398 ff.), fluidized beds are of advantage when temperature control is a particular problem in the reaction or where catalyst coking is a particular problem, i.e., where catalyst regeneration must be facilitated.
In the past, fluidized or otherwise movable beds have been used in conjunction with the underlying reactions involved in this invention only when amorphous aluminosilicate catalysts are employed since these have coking problems attendant to their use. See, e.g., U.S. Pat. Nos. 2,807,618 (column 1); 3,946,020 (column 2); 4,147,874 (column 2); 4,149,002 (column 2); DT No. 2,203,384; and DS No. 1,670,514, among many others. Fluidized bed reactors have not been employed in the reactions involved herein using the required crystalline catalysts since the latter are known to be associated with exceptionally low coke deposits. (See, e.g., Dejaifve et al, Journal of Catalysis 70, 123-136 (1981); Walsh et al, Journal of Catalysis 56, 195-197 (1979); Cormerais et al, Zeolites, 1981, Vol. 1, October, 141-144; Derouane et al, Applied Catalysis, 1 (1981) 201-224; E. G. Derouane, Catalysis by Zeolites, B. Imelik et al. (Editors), 1980, Elsevier Scientific Publishing Company, Amsterdam, 5-46; all of whose disclosures are entirely incorporated by reference herein as they relate to the catalysts employed in this invention.)
Another factor in the prior art use of fixed beds is that only the reactions of this invention are known to be relatively temperature insensitive. See, e.g., examples 6 and 10 below. In this regard, see also U.S. Pat. No. 4,071,573 which employs the same crystalline aluminosilicate catalysts mentioned above in a different reaction but in a unique fluidized bed design since the reactions involved have associated therewith a need for disposal of the exothermic heat of reaction which otherwise is detrimental, especially to the catalyst lifetime. See also U.S. Pat. Nos. 3,894,106 and 3,894,107 each of which discloses the equivalent use of such crystalline catalysts in fixed or fluidized beds in conjunction with different reactions. Note, in particular, column 7, lines 23-32 and column 9, lines 50-64 respectively.
As can be seen, as is generally the case, except for specific system considerations which might dictate a preference for fixed versus fluidized or otherwise movable beds in a given reaction, in general, the two are expected to produce equivalent results. Particularly with reference to the class of reactions to which this invention relates, the two types of beds have been shown to produce essentially equivalent results in the past. This is not unusual. Similar equivalencies have been observed in other systems. For example, in the ammoxidation of propene to acrylonitrile, Barbouteau et al (Chem. Eng. J. (Lausaune), 20, 43 1980); CA: 943737x), found that a fluid bed having a tube of a diameter twice that of a fixed bed reactor produced comparable results. The same research group investigated the oxidation of butane to maleic anhydride and found that under a narrow range of conditions, the fluid bed gave better selectivities than a fixed bed; however, under the broader range of conditions, the results were comparable. (Laguerie et al, Chem. Eng. J. (Lausaune) 5, 33 (1973); CA: 79:20928t.) As another example, the catalytic cracking of gas-oil was examined in the past, and catalyst life and selectivity were shown to be equivalent while conversion was always higher in the fixed bed. Gross et al in Ind.Eng.Chem. Process R.Dev., 13, 199 (1974); CA 82:100966.
Thus, for the reaction to which this invention relates, there is a prior art expectancy that fixed and fluidized or otherwise movable beds will give essentially equivalent results when a crystalline aluminosilicate zeolite catalyst is employed, i.e., that the desired yields cannot be improved by carrying out the prior art fixed bed reaction in a fluidized or otherwise movable bed.